Perpendicular magnetic anisotropy (PMA) spin-RAM have advantages such as an ability of scalability, a low power consumption, a high performance and a high reliability, and thus is very likely to become a mainstream technique of the next-generation new non-volatile memory. However, to enhance a magnetoresistance ratio of devices and to reduce write currents, a free layer still has to be mainly composed of films of perpendicularly magnetized CoFeB or CoFe. Nonetheless, since such free layer has a low equivalent magnetic anisotropy coefficient (Keff) and a small thickness, a thermal stability thereof is low; thus, the free layer fails to be a non-volatile memory. Therefore, it is proposed lately in literature that improvements to a thermal stability of the CoFeB or the CoFe free layer may be enhanced by using double layer of MgO. However, a process temperature thereof needs to be lower than a temperature for CMOS back-end-of-line (BEOL) process, otherwise a device characteristic may suffer a degradation. Accordingly, how to improve a heat resistance for the magnetic tunnel junction device with perpendicular magnetization is now an important issue to be solved in the field.